My understanding of "self-defense" has been influenced by this:
Over time, this understanding was reinforced by assaults on coworkers and friends. I am fortunate in what the assaults on my person have not resulted in serious injury:
1. Classic "stick 'em up" robbery, while working as a pizza delivery driver. I felt something on my back and was told in rude language to set the pizzas and 2-liter soda bottle down, hand over my wallet, and walk back to my car - all without turning around or he would f'n kill me on the spot. So I have no idea what my robber looked like.
2. Angry middle-aged man charging through crowded subway train. Nothing to do about that.
3. Angry middle-aged man #2 attempting to elbow me in the face as he exited the train. I let his elbow slide off my arm. He screamed in colorful language that I was in his way. Perspective is a funny thing - the train was crowded and I was pressing myself away from the doorway so he could get out. The rest of us laughed.
4. Drunk guy grabbed my arm with two hands (classic Aikido attack) and pulled me into a mosh pit. I tried pulling away but he was stronger and I lost the tugging contest.
4. Drunk guy #2 stared at me as I got into line to order from a DC Slices food truck. He continued staring, then suddenly threw a round kick at my shoulder. His foot felt as light as a feather - well his balance was pretty bad.
So the first rule of self defense is to not be in a place where one would have to defend oneself. Well, I guess I couldn't follow it for any of the above scenarios. For most of my career here I have depended on public transportation. In the case of my coworker who got beat up on an enclosed section of a bike trail by a group of teenagers, I would have turned around and gone the other way if I saw that group blocking the bike trail.
Next rule is be aware of who is around you. Admittedly I wasn't paying attention to either drunk guy. And when you're in a crowded train, it's hard to identify who might be a threat. Criminals usually don't attack in crowded trains. Unfortunately in this area, lots of people walk by themselves looking at their mobile devices and/or listening to music on headphones/earbuds. It's too easy to follow one of them to a place where he/she is alone and then attack by surprise.
Aikido only comes into play after you've observed the first two rules and you're still being attacked.
Of course, you have to know that you're being attacked. This is where the No Nonsense Self Defense site is useful - it describes various clues you can look for to identify a potential attacker. If the attacker is not launching a surprise attack, he/she will go for an "interview" to determine if you are a desirable victim. BTW, everyone I know who was assaulted in this area was assaulted by surprise attack, some by a variation of the "Knockout Game", a messed up "game" in which a bored teen picks victim at random and tried to knock out the victim with one strike.